The United Nations peacekeeping chief yesterday announced that the big showdown in Aleppo between the Syrian Army and rebel forces is imminent, as hopes for international mediation in Syria sank to their lowest level yet with the resignation of the diplomat charged with implementing a peace plan.

The Army and rebels have been fighting fierce, small-scale clashes in Aleppo for three weeks as both sides ramp up for an all-out battle for the city.

Cellphone service to the city has been cut off, making it difficult to discern fully what is happening on the ground, but Army planes have been shelling parts of the city and rebels have seized tanks from a military base, turning them on regime troops. Capt. Ammar Al-Wawi of the Free Syrian Army told CNN that the Army has also been using fighter jets on sections of the city.

"The increasing militarization on the ground and the clear lack of unity in the Security Council have fundamentally changed the circumstances for the effective exercise of my role," he said. "The bloodshed continues, most of all because of the Syrian government's intransigence and continuing refusal to implement the six-point plan, and also because of the escalating military campaign of the opposition—all of which is compounded by the disunity of the international community."

He is not the only world leader who seems to be done with trying to work out a solution to the Syrian conflict through the proper international channels: Security Council President Gerard Araud yesterday said that the group should turn its focus to humanitarian issues, CNN reports. "The divisions in the council are very deep. I think its irreconcilable in political terms," he said. "Maybe we should work on [the] humanitarian situation, which is becoming disastrous. At least the Security Council would be useful," Mr. Araud added.

Mr. Annan should not be blamed for his failure to bring about any progress in Syria, the Guardian argues in an editorial. The so-called Annan peace plan, backed strongly by council members, it says, was doomed to fail from the start because neither the Army nor the increasingly militarized opposition ever seriously considered pausing their assault. There is no point in Annan's successor picking up its mantle, the Guardian writes.

Try as one might to concoct a scenario where the six-point Annan plan for a negotiated political transition may one day be resuscitated – a successor will be found as envoy and, as Annan says, the plan is the security council's, not his – it is hard to avoid the conclusion that his resignation marks the end of diplomacy in Syria. It is a landmark moment.

Some argued almost from the start that Annan, one of the world's most seasoned diplomats, only made things worse. The means, in the form of a ceasefire, offered loyalist troops a respite, while the end, a political process became a free get-of-jail-card for Bashar Assad. The Annan plan, it was said, exemplified the gap between expectation and delivery that Assad exploited with murderous intent.

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The undoing of the Annan plan surely lay not so much with the plan itself – there never was any other real alternative to stop a brutally repressed civilian insurrection turning into a civil war – but in the UN security council that approved it.

Anthony Cordesman, an analyst at the Center for Strategic and International Studies in Washington, said the trouble with Annan's peace plan, which included a cease-fire and a political transition plan that would include some members of the Assad regime in the next government, is that it "came too late." Compounding that was the "mounting feud" between the Western council members – US, Britain, and France – and Russia, which had the two sides vociferously opposing each other at the council, The Wall Street Journal reports.

"I don't believe that Kofi Annan can be blamed for hoping that Assad might have behaved differently," Mr. Cordesman said. "But the fact is that this entire effort came late. The whole idea that observers could keep the peace was very, very optimistic from the start."

CBS News correspondent Charlie D'Agata, reporting from the outskirts of Aleppo, reports that "the rebels take very little interest in what is going on at the UN. The fighters on the front lines in the battle to wrest Syria's largest city definitively from the control of Assad's regime say the UN has proven itself of little help to their cause."